Lunar knot

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The nodes of the moon
The inclination is shown exaggerated, it is in reality about 6 °

The lunar nodes or dragon points are the intersections of the lunar orbit with the ecliptic plane :

  • the ascending lunar node (common symbol: ☊) is the one where the moon changes from the southern to the northern side of the ecliptic ("ascends")
  • the descending lunar node  (☋) is the one where the moon changes from the north to the south side of the ecliptic.

The connection between the two nodes is called the knot line.

The draconian month (drakon / δράκων: Greek for dragon) is the time span between two successive passages of the moon through the ascending knot .

Nodes and eclipses

Dragon in the Astronomicum Caesareum. (1540) by Peter Apian
Astronomicum Caesareum.jpg

Eclipses are only possible in the vicinity of a passage of the moon through one of the nodes:

Both types of darkness can occur at both the ascending and the descending node.

The time interval between two passes of the sun through the same lunar node is called the eclipse year .

The old expression dragon point is derived from the image of the dragon , which swallows the sun or moon in a darkness ( Latin draco "dragon"). The lunar nodes were also known as

  • Dragon's head ( Caput draconis , ascending lunar knot)
  • Dragon tail ( Cauda draconis , descending lunar knot)

and had a special meaning.

The two points of the lunar orbit with the greatest northern and southern ecliptical latitudes (i.e. the greatest distance from the ecliptic) were called the dragon's belly ( Venter draconis ); they differ from the nodes by 90 ° in length .

The same etymological meaning is in the "ecliptic" (Greek ékleipsis - literally "concealment, extinction"). It can also be found in Sino-Confucian mythology in Xi and He , 羲 和, the heavenly officials for the sun and moon. However, this myth has no known connection with the constellation "Dragon" .

The dragon point also names the draconian period , i.e. the general period of two passages of a celestial object through its nodes.

The draconian month

The length of the draconian month is given as 27.21222 days. That's 27 days, 5 hours, 5 minutes and 35.8 seconds.

It is thus 0.1095 days (i.e. 2h 37.6m) shorter than the tropical month - related to the equinox - because the lunar nodes move backwards by approx. 19.3 ° annually in the ecliptic due to the precession of the lunar orbit plane . A complete orbit of the nodes in the ecliptic takes 6798 days or 18.61 years.

The draconian month is used to calculate eclipse cycles , the regular repetitions of solar and lunar eclipses.

Lunar knot and nutation

The lunar nodal drift causes a change in the position of the lunar orbit on an observer's sphere .

If one of the lunar nodes coincides with the spring equinox or with the autumn equinox , the lunar orbit becomes extreme. In this case, they add up

to a maximum declination of the moon of 28 ° 36 '(minimum 18 ° 17'). The extremes of this cycle are called the moon turns .

Specifically, the following inclinations of the lunar orbit towards the celestial equator result for the coincidence of a lunar node with the spring or autumn point:

Spring equinox Autumn point
before: -28 ° 36 '
after: + 28 ° 36'
before: + 18 ° 17 '
after: -18 ° 17'
before: -18 ° 17 '
after: + 18 ° 17'
before: + 28 ° 36 '
after: -28 ° 36'

The gravitational action of the moon, which fluctuates around these extreme positions, causes nutation , a periodic axis fluctuation of the earth with the same duration of 18.613 years, which is therefore called the nutation period.

See also


Individual evidence

  1. Joachim Herrmann: dtv atlas for astronomy (= dtv 30069). 2nd Edition. Deutscher Taschenbuch-Verlag, Munich 1974, ISBN 3-423-03006-2 , p. 51.